Accelerometers are required in numerous applications, such as navigation, guidance, microgravity measurements, seismology and platform stabilization. Also, as they become manufacturable at low cost with small size, they attain a large potential consumer market in their application as a GPS-aid to obtain position information when the GPS receivers lose their line-of-sight with the satellites.
Some accelerometers are fabricated by surface micromachining or bulk micromachining. The surface micromachined devices are fabricated on a single silicon wafer. However, they generally have low sensitivity and large noise floor, and thus cannot satisfy requirements of many precision applications.
Some high resolution accelerometers are bulk micromachined and use multiple wafer bonding as part of their manufacturing process. This wafer bonding is a complex fabrication step, and hence results in lower yield and higher cost. Also, forming damping holes in the thick bonded wafers is difficult, and thus special packaging at a specified ambient pressure is typically needed to control the device damping factor. Finally, due to wafer bonding, these devices show higher temperature sensitivity and larger drift.
U.S. Pat. No. 5,345,824 discusses a monolithic capacitive accelerometer with its signal conditioning circuit fabricated using polysilicon proofmass and surface micromachining.
U.S. Pat. No. 5,404,749 discusses a borondoped silicon accelerometer sensing element suspended between two conductive layers deposited on two supporting dielectric layers.
U.S. Pat. No. 5,445,006 discusses a self-testable microaccelerometer with a capacitive element for applying a test signal and piezoresistive sense elements.
U.S. Pat. No. 5,461,917 discusses a silicon accelerometer made of three silicon plates.
U.S. Pat. No. 5,503,285 discusses a method for forming an electrostatically force rebalanced capacitive silicon accelerometer. The method uses oxygen implantation of the proofmass to form a buried oxide layer and bonding of two complementary proofmass images together. The implanted oxide layer is removed after bonding to form an air gap and release the proofmass.
U.S. Pat. No. 5,535,626 discusses a capacitive microsensor formed of three silicon layers bonded together. There is glass layer used between each two bonded silicon pairs.
U.S. Pat. No. 5,540,095 discusses a monolithic capacitive accelerometer integrated with its signal conditioning circuitry. The sensor comprises two differential sense capacitors.
U.S. Pat. No. 5,559,290 discusses a capacitive accelerometer formed of three silicon plates, attached together using a thermal oxide interface.
U.S. Pat. No. 5,563,343 discusses a lateral accelerometer fabricated of a single crystal silicon wafer.
U.S. Pat. No. 5,605,598 discloses a monolithic micromechanical vibrating beam accelerometer having a trimmable resonant frequency and method of making same.
The paper entitled "Advanced Micromachined Condenser Hydrophone" by J. Bernstein et al, Solid-State Sensor and Actuator Workshop, Hilton Head, S.C., June, 1994, discloses a small micromechanical hydrophone having capacitor detection. The hydrophone includes a fluid-filled variable capacitor fabricated on a monolithic silicon chip.
The paper entitled "High Density Vertical Comb Array Microactuators Fabricated Using a Novel Bulk/PolySilicon Trench Refill Technology", by A. Selvakumar et al., Hilton Head, S.C., June 1994, discloses a fabrication technology which combines bulk and surface micromachining techniques. Trenches are etched and then completely refilled.
Numerous U.S. patents disclose electroplated microsensors such as U.S. Pat. Nos. 5,216,490; 5,595,940; 5,573,679; and 4,598,585.
Numerous U.S. patents disclose accelerometers such as U.S. Pat. Nos. 4,483,194 and 4,922,756.
U.S. Pat. No. 5,146,435 discloses an acoustic transducer including a perforated plate, a movable capacitor plate and a spring mechanism, all of which form a uniform monolithic structure from a silicon wafer.